The Qinghai-Tibet plateau is one of China’s highest risk areas for human plague, caused by the infectious bacterium Yersinia pestis. The main carrier of the disease is the Himalayan marmot, a rodent about the size of a large domestic cat. Since the first marmot death caused by the human plague strain of Y. pestis (biovar antiqua) was documented in 1954, a total of 468 human plague cases with 240 deaths have been reported. In a new study published in PLOS Neglected Tropical Diseases, researchers found that human plague likely spread from Tibetan sheep, who make up about one-third of China’s total sheep population, and in turn these sheep likely contracted the disease from marmots.

To better understand local outbreaks, Wei Li of the CDC in China and colleagues, extracted the genomic DNA from 38 strains of Y. pestis isolated from Tibetan sheep, Himalayan marmots, and humans. Y. pestis isolated from Tibetan sheep and local marmots all were found to be of the biovar antiqua strain. The researchers used a phylogenetic tree, used like a family tree to track evolutionary lineages, to reconstruct the Y. pestis strain’s evolutionary path in the region. Researchers determined the disease originally transferred from Himalayan marmot to sheep to human. In addition, the genomic analysis of the Tibetan sheep-related strains revealed the strains have territory-specific characteristics: when there is no geographic barrier between adjacent areas, the pathogens isolated from adjacent areas were evolutionarily closely related.

The exact pathway of bacterial transmission still needs further study. Researchers suggest that transmission to Tibetan sheep could occur not only via fleas, but also when the sheep lick infected dead rodents, which they may do to obtain micronutrients. Transmission from sheep to humans is linked with the skinning and butchering of sheep as well as with consumption of undercooked meat.

“To the best of our knowledge,” the researchers note, “natural infection of sheep with Y. pestis is rare elsewhere in the world. The Tibetan sheep plague has some novel features, such as transmission across long distances. Therefore, the hazards of Tibetan sheep plague should not be underestimated.”